Fluoride will be added to tap water in Southampton after health bosses voted through the plans despite protests.
It is the first time a health trust in England, rather than water companies, has been allowed to introduce fluoridation under new laws.
The idea has proved controversial with 72% of 10,000 respondents in a public consultation opposing the plan.
Despite the opposition, the South Central Strategic Health Authority (SCSHA) unanimously backed the move.
Southampton City Council was in favour, but Hampshire County Council was against the plan, which is designed to cut tooth decay and which will affect 200,000 people.
Jim Easton, the SCSHA chief executive, said: "We recognise that water fluoridation is a contentious issue for some people.
"The board was satisfied that, based on existing research, water fluoridation is a safe and effective way to improve dental health."
The SCSHA will now write to Southern Water instructing the firm to increase levels of fluoride from a natural 0.08 part per million to one part per million.
The Southampton City Primary Care Trust (PCT) said more than 40% of children in the area were suffering from tooth decay and the increase was desperately needed.
Beeb.
This is interesting; I didn't know that the NHS had the power to introduce fluoridation.